This invention relates to superconducting magnet assembly for a magnetic resonance imaging system (hereinafter called "MRI"), and more particularly to compensation for magnetic inhomogeneities through control. of flow in correction coils in the magnet.
As is well known, a superconducting magnet can be made superconducting by placing it in an extremely cold environment, such as by enclosing it in a cryostat or pressure vessel containing liquid helium or other cryogen. The extreme cold ensures that the magnet coils are superconducting, such that the coils can be operated in persistent mode, that is, a power supply can be connected for a short time to start current flowing through the coils, then a superconducting switch can be closed, the power supply removed, and the current will continue to flow, thereby maintaining the coils current and resultant magnetic field. Superconducting magnets find wide application in the field of MRI.
In a typical MRI magnet, the main superconducting magnet coils are enclosed in a cylindrical cryogen pressure vessel, contained within a vacuum vessel and forming an imaging bore in the central region. The main magnet coils develop a strong magnetic field in the imaging bore.
However, it is necessary for acceptable quality imaging to provide and maintain a strong homogeneous magnetic field in the imaging region. Typical field strengths of 0.5 to 1.5 T require homogeneity of 10 parts per million (ppm) over volumes on the order of a 45 cm diameter sphere. While magnets can be designed to produce these field strengths and this level of homogeneity, achievable manufacturing tolerances are too large to guarantee that the manufactured magnet will meet the 10 ppm homogeneity requirements. One method of achieving high magnetic field strength and high homogeneity is to build the magnet of superconducting wires and add magnetic "shimming". One method to "shim" the magnet is to add small amounts of magnetic material (called shims) such as ferromagnetic low carbon steel to appropriate places near the imaging volume. While this method typically saves magnet cost, the additional ferromagnetic material has unwanted effects including force. As more magnetic material is added, the force on the magnetic material increases. Another unwanted effect is that the magnetic properties of magnetic material shims are temperature dependent such that as the amount of magnetic material increases, the unwanted temperature, effects increase.
Another shimming method is to add superconducting correction coils to the magnet. The currents in these correction coils can then be adjusted after the magnet has completed manufacture to "shim" the magnet to the required homogeneity. However, superconducting wire must be maintained at cryogenic temperatures, and present arrangements for adjusting the current in the correction coils add significant, unwanted cost and complexity to the magnet. In addition to the unwanted cost and complexity, mechanical leads which extend from the outside of the superconducting magnet to the interior of the cryogen vessel also add a thermal load to the refrigerators that recycle the helium gas produced back to liquid helium to maintain the superconducting magnet at cryogenic temperatures.